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The temperature-density equation of Kinetic Theory and the conservative neutron transport equation are studied. In both cases a modified version of the Larsen-Habetler resolvent integration technique is applied to obtain full-range and half-range expansions. For the neutron transport equation the method applied is seen to have notational advantages over previous approaches. In the case of the temperature-density equation this development extends previous results by enlarging the class of expandable functions and has the added advantage of rigor and simplicity. As a natural extension of the Kinetic Theory results, an integral equation for the surface density is derived for half-space problems involving the boundary condition of arbitrary accommodation. / Ph. D.

In this work, turbulent premixed combustion was analyzed through CH (methylidyne) planar laser induced fluorescence (PLIF). Flame topography measurements in terms of flame surface density and curvature were calculated based on the flame front detected by the CH PLIF signal. The goal of this work was to investigate turbulent flames with extremely high turbulence intensity using a recently developed HiPilot burner (a Bunsen-type burner). The studies were first conducted on a series of piloted jet flames to validate the methodology, and then conducted on the highly turbulent flames generated by the HiPilot burner. All flames were controlled by combusting methane and air under a fuel to air equivalence ratio of Φ=1.05, and the Reynolds number varied from 7,385 to 28,360. Flame surface density fields and profiles for the HiPilot burner are presented. These flame surface density measurements showed an overall decrease with height above the burner. In addition, curvature statistics for the HiPilot flames were calculated and probability density functions of the curvature samples were determined. The probability density functions of curvature for the flames showed Gaussian-shaped distributions centered near zero curvature. To conclude, flame topography measurements were verified on jet flames and were demonstrated on the new HiPilot flames. / Master of Science / Optical diagnostics are powerful techniques that enable the study of turbulent flames without physical interruption. The optical diagnostic technique in this thesis implemented planar laser induced fluorescence. In planar laser induced fluorescence, a laser is used to excite a specific molecular species present within a two-dimensional plane in the flame. The excited species releases the extra energy by emission of light which is the signal captured on a camera. One useful purpose of using optical diagnostics, such as planar laser induced fluorescence, is the ability to image the flame structure of turbulent flames. The flame structure is significant for two reasons. First, the flame structure details how the chemistry of the flame interacts with the turbulent flow field. Second, the flame structure is directly related to the burning rate of the reactants. The primary contribution of this thesis investigated the two-dimensional flame structure of a newly designed burner named the HiPilot burner. However, in order to strengthen the fidelity of the methods for determining certain flame structure quantities a precursive analysis on the classical jet flame was completed. The results acquired show structural measurements of the HiPilot flames which contribute to the repository of data for the combustion community

The influence of stratification on the structure of turbulent methane/air combustion is investigated using experimental data from laboratory scale burners: a weakly turbulent slot burner, and a higher turbulence co-annular swirl burner. The degree of stratification can be controlled independently of the overall fuel/air flow rate. The resulting measurements of scalar and velocity fields provide detailed test cases for existing and emerging turbulent flame models, covering a range of u'/sL from 1 to 10, turbulence intensities from 5% to 60%, and stratification ratios from 1 to 3. Simultaneous Rayleigh/Raman/CO-LIF measurements of temperature and major species concentrations - CH4, CO2, CO, H2, H2O and O2 - along a line are used to investigate the structure of a series of flames in both the slot and swirl burners. Concurrent cross-planar OH-PLIF allows thermal gradients to be angle corrected to their three-dimensional values. Finally, non-reacting and reacting velocity fields complete the flame database. The behavior of major species concentrations in the slot and swirl burner with respect to temperature is found to agree well on the mean with unstrained premixed laminar flame calculations. Scalar means conditioned on stoichiometry also show good agreement, aside from hydrogen which is enhanced under stratified conditions. Surface density function and scalar dissipation are lower than calculated values in all cases, suggesting that turbulence-induced thickening dominates the effect of increased strain. Metrics commonly used to derive flame surface density (FSD) were investigated. FSD may be determined using a statistical method based on measurements of temperature and its gradient, or a geometric method based on 2D temperature or LIF imaging. A third metric, an extension of the geometric method, is proposed. Good agreement is observed between the three metrics. The current database provides the first detailed high resolution scalar measurements for premixed and stratified flames. The data analysis provides insight into the physics of stratification: for the flames considered, the effects of stratification appear to be surprisingly small compared to those of turbulence, even at significant stratification ratios. The datasets provide a means of validating current and future computational turbulent combustion models.

In the last 10-15 years, large-eddy simulation (LES) has become well established for non-reacting flows, and several successful models have been developed for the transfer of momentum and kinetic energy to the subfilter-scales (SFS). However, for reacting flows, LES is still undergoing significant development. In particular, for many premixed combustion applications, the chemical reactions are confined to propagating surfaces that are significantly thinner than the computational grids used in practical LES. In these situations, the chemical kinetics and its interaction with the turbulence are not resolved and must be entirely modelled. There is, therefore, a need for accurate and robust physical modelling of combustion at the subfilter-scales. In this thesis, modelled transport equations for progress variable and flame surface density (FSD) were implemented and coupled to the Favre-filtered Navier-Stokes equations for a compressible reactive thermally perfect mixture. In order to reduce the computational costs and increase the resolution of simulating combusting flows, a parallel adaptive mesh (AMR) refinement finite-volume algorithm was extended and used for the prediction of turbulent premixed flames. The proposed LES methodology was applied to the numerical solution of freely propagating flames in decaying isotropic turbulent flow and Bunsen-type flames. Results for both stoichiometric and lean flames are presented. Comparisons are made between turbulent flame structure predictions for methane, propane, hydrogen fuels, and other available numerical results and experimental data. Details of subfilter-scale modelling, numerical solution scheme, computational results, and capabilities of the methodology for predicting premixed combustion processes are included in the discussions. The current study represents the first application of a full transport equation model for the FSD to LES of a laboratory-scale turbulent premixed flame. The comparisons of the LES results of this thesis to the experimental data provide strong support for the validity of the modelled transport equation for the FSD. While the LES predictions of turbulent burning rate are seemingly correct for flames lying within the wrinkled and corrugated flamelet regimes and for lower turbulence intensities, the findings cast doubt on the validity of the flamelet approximation for flames within the thin reaction zones regime.

O IPEN-CNEN/SP produz o combustível para suprir o seu reator nuclear de pesquisas IEA-R1. O combustível é montado a partir de placas combustíveis contendo um núcleo do compósito U3Si2-Al. Uma boa homogeneidade na distribuição de urânio no núcleo da placa combustível é essencial, pois garante um bom desempenho sob irradiação. Considerando a baixa potência do reator IEA-R1, atualmente, a distribuição de urânio na placa combustível é avaliada apenas por meio de inspeção visual de radiografias. Contudo, tendo em vista a possibilidade do IPEN fabricar o combustível para o novo Reator Multipropósito Brasileiro (RMB), que terá potência elevada, tornou-se inadiável o desenvolvimento de uma metodologia para determinar quantitativamente a homogeneidade da distribuição de urânio no combustível. Este trabalho apresenta uma metodologia baseada na atenuação de raios X para quantificar a distribuição da concentração de urânio no núcleo da placa combustível, por meio da análise da densidade óptica de radiografias e comparação com padrões. Os resultados demonstraram a não aplicabilidade do método, considerando a especificação atual para as placas combustíveis, devido ao alto valor do erro intrínseco ao método. Contudo, o estudo dos erros envolvidos na metodologia, buscando aumentar a sua exatidão e precisão, pode viabilizar a aplicação do método para qualificar o produto final. / IPEN/CNEN-SP produces the fuel to supply its nuclear research reactor IEA-R1. The fuel is assembled with fuel plates containing an U3Si2-Al composite meat. A good homogeneity in the uranium distribution inside the fuel plate meat is important from the standpoint of irradiation performance. Considering the lower power of reactor IEA-R1, the uranium distribution in the fuel plate has been evaluated only by visual inspection of radiographs. However, with the possibility of IPEN to manufacture the fuel for the new Brazilian Multipurpose Reactor (RMB), with higher power, it urges to develop a methodology to determine quantitatively the uranium distribution into the fuel. This paper presents a methodology based on X-ray attenuation, in order to quantify the uranium concentration distribution in the meat of the fuel plate by using optical densities in radiographs and comparison with standards. The results demonstrated the inapplicability of the method, considering the current specification for the fuel plates due to the high intrinsic error to the method. However, the study of the errors involved in the methodology, seeking to increase their accuracy and precision, can enable the application of the method to qualify the final product.

O IPEN-CNEN/SP produz o combustível para suprir o seu reator nuclear de pesquisas IEA-R1. O combustível é montado a partir de placas combustíveis contendo um núcleo do compósito U3Si2-Al. Uma boa homogeneidade na distribuição de urânio no núcleo da placa combustível é essencial, pois garante um bom desempenho sob irradiação. Considerando a baixa potência do reator IEA-R1, atualmente, a distribuição de urânio na placa combustível é avaliada apenas por meio de inspeção visual de radiografias. Contudo, tendo em vista a possibilidade do IPEN fabricar o combustível para o novo Reator Multipropósito Brasileiro (RMB), que terá potência elevada, tornou-se inadiável o desenvolvimento de uma metodologia para determinar quantitativamente a homogeneidade da distribuição de urânio no combustível. Este trabalho apresenta uma metodologia baseada na atenuação de raios X para quantificar a distribuição da concentração de urânio no núcleo da placa combustível, por meio da análise da densidade óptica de radiografias e comparação com padrões. Os resultados demonstraram a não aplicabilidade do método, considerando a especificação atual para as placas combustíveis, devido ao alto valor do erro intrínseco ao método. Contudo, o estudo dos erros envolvidos na metodologia, buscando aumentar a sua exatidão e precisão, pode viabilizar a aplicação do método para qualificar o produto final. / IPEN/CNEN-SP produces the fuel to supply its nuclear research reactor IEA-R1. The fuel is assembled with fuel plates containing an U3Si2-Al composite meat. A good homogeneity in the uranium distribution inside the fuel plate meat is important from the standpoint of irradiation performance. Considering the lower power of reactor IEA-R1, the uranium distribution in the fuel plate has been evaluated only by visual inspection of radiographs. However, with the possibility of IPEN to manufacture the fuel for the new Brazilian Multipurpose Reactor (RMB), with higher power, it urges to develop a methodology to determine quantitatively the uranium distribution into the fuel. This paper presents a methodology based on X-ray attenuation, in order to quantify the uranium concentration distribution in the meat of the fuel plate by using optical densities in radiographs and comparison with standards. The results demonstrated the inapplicability of the method, considering the current specification for the fuel plates due to the high intrinsic error to the method. However, the study of the errors involved in the methodology, seeking to increase their accuracy and precision, can enable the application of the method to qualify the final product.

In this work pentacene adsorption on both flat and stepped Ag(111) surfaces were investigated by using Density Functional Theory within Projected Augmented Wave method. On the flat Ag(111) surface favorable adsorption site for a single pentacene molecule was determined to be the bridge site with an angle of 60&amp / #9702 / between pentacene molecular long axis and [011] lattice direction. Potential energy surface was found to be flat, especially along lattice directions. Diffusion and rotation barriers for pentacene on this surface were found to be smaller than 40 meV indicating the possibility of a two dimensional gas phase. Calculated adsorption energies for the flat surface indicate a weak interaction between molecule and the surface indicating physisorption. On the flat surface monolayer case is found to have lower adsorption energy than the isolated case due to pentacene&amp / #8722 / pentacene interactions. On the stepped Ag(233) surface, close to the step edge, adsorption energy increased significantly due to the stronger interaction between pentacene molecule and low coordinated silver step atoms. On the terraces of this surface, far from step edges, however a flat potential energy surface was observed similar to the case of flat Ag(111) surface. On the stepped surface pentacene found its favorable configuration as parallel to the step with a tilt angle similar to the observed thin film phase of pentacene on Ag(111) surface. Pentacene molecule showed small distortions on stepped surface and are closer to the silver step atoms 1 &Aring / more than the case of flat surface, hinting a chemical interaction as well as van der Waals interactions. However on Ag(799) surface, the perpendicular orientation of the pentacene molecule to the step direction showed no strong interaction due to less matching of carbon atoms with silver step atoms.

In the last 10-15 years, large-eddy simulation (LES) has become well established for non-reacting flows, and several successful models have been developed for the transfer of momentum and kinetic energy to the subfilter-scales (SFS). However, for reacting flows, LES is still undergoing significant development. In particular, for many premixed combustion applications, the chemical reactions are confined to propagating surfaces that are significantly thinner than the computational grids used in practical LES. In these situations, the chemical kinetics and its interaction with the turbulence are not resolved and must be entirely modelled. There is, therefore, a need for accurate and robust physical modelling of combustion at the subfilter-scales. In this thesis, modelled transport equations for progress variable and flame surface density (FSD) were implemented and coupled to the Favre-filtered Navier-Stokes equations for a compressible reactive thermally perfect mixture. In order to reduce the computational costs and increase the resolution of simulating combusting flows, a parallel adaptive mesh (AMR) refinement finite-volume algorithm was extended and used for the prediction of turbulent premixed flames. The proposed LES methodology was applied to the numerical solution of freely propagating flames in decaying isotropic turbulent flow and Bunsen-type flames. Results for both stoichiometric and lean flames are presented. Comparisons are made between turbulent flame structure predictions for methane, propane, hydrogen fuels, and other available numerical results and experimental data. Details of subfilter-scale modelling, numerical solution scheme, computational results, and capabilities of the methodology for predicting premixed combustion processes are included in the discussions. The current study represents the first application of a full transport equation model for the FSD to LES of a laboratory-scale turbulent premixed flame. The comparisons of the LES results of this thesis to the experimental data provide strong support for the validity of the modelled transport equation for the FSD. While the LES predictions of turbulent burning rate are seemingly correct for flames lying within the wrinkled and corrugated flamelet regimes and for lower turbulence intensities, the findings cast doubt on the validity of the flamelet approximation for flames within the thin reaction zones regime.

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